[Intel-gfx] [PATCH 1/2] drm/i915: Mark CPU cache as dirty on every transition for CPU writes

[Chris Wilson]

On Fri, Apr 28, 2017 at 03:55:56PM -0700, Dongwon Kim wrote:
> Hi Chris,
> 
> I tried this but I still see tests are failing. 
> I wanted to debug it little further to find a specific
> condition where clflush is missing but didn't have 
> enough time. I will look into this early next week.

Did you check this patch separately?

So we are still missing a transition where we need to flag the cache as
becoming dirty. And I still believe you have a
"set-cache-level(snooped); gpu write; set-cache-level(none); gpu access"
sequence. 

This patch should be marking as any write to a snooped bo as making the
cache-dirty. So we should be caching any and all transitions from snoop
to none, as that cache_dirty flag will not go away until we clflush.

And the real active ingredient of this patch is to always flush the
dirty_cache before rendering, not just if the object was in the CPU
write domain at that time.

Hmm, one thing to check is that if your userspace is not declaring some
domain access that is dirtying the cache. Or if you are using mocs that
override the cache tracking, without adjusting the PTE. If you are doing
the latter, there isn't much the kernel can do to help.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre